Electrolytic process for preparing d-ribose



United States Patent Ofilice 3 ,3 12,508 Patented Apr. 4, 1967 FOR PREPARING This invention relates to improved process for preparing D-ribose by the electrolytic reduction of D-ribonolactone.

D-ribose is an useful intermediate for the synthesis of vitamin B It has been prepared commercially by the reduction of D-ribonolactone as shown by the following Either chemical or electrolytic methods have been employed for this reduction. The chemical reduction Was reported, eg in Helvetica Chimica Acta, vol. 19, p. 189 (1936) in which sodium amalgam is used as the reducing agent. But, this method requires a considerable amount of mercury, and the preparation of amalgam is troublesome. Moreover, the adjustment of the pH in the reaction mixture is difiicult.

The electrolytic reduction is advantageous, as compared to the chemical reduction. The electrolytic methods which were repotred heretofore, are all methods which comprise using mercury as the cathode, and an alkali metal salt of an inorganic acid as the electrolyte which is able to form amalgam with the mercury. (e.g. J. Chem. Soc., Japan, vol. 71, p. 310 (1950), Pat. Publ. No. 4,359 (1950) Japan). According to these methods, the yields of D-ribose are higher than the chemical reduction While the amounts of mercury required are smaller. (Usually /s% of the chemical reduction.) But, because the electrolyte is an alkali metal salt of an inorganic acid, there remains such defects as follows:

In the course of electrolysis, an alkali metal ion discharges on the surface of cathode and forms the amalgam with mercury continuously. The amalgam thus formed tends to react with water secondarily and produces an alkali metal hydroxide on the surface of cathode. As a result, the cathode is covered by a layer of alkali during the electrolysis, even if the electrolytic solution itself is kept in a weakly acidic condition with boric acid. On the other hand, D-ribonolactone is unstable in a strongly alkaline condition, and its lactone ring is cleaved to produce alkali metal ribonate Which is no longer reduced. Accordingly, a part of D-ribonolactone is wasted by this unfavorable side-reaction, decreasing the yield of D- ribose.

We have found that the electrolytic reduction of D- ribonolactone may be improved by employing inorganic ammonium salts instead of alkali metal salts, as an electrolyte. According to this improved method, the yield of D-ribose may be increased, since the cleavage of D- ribonolactone may be substantially prevented.

According to our invention, said electrolysis may be carried out in a standard electrolytic cell, for example, the electrolytic cell which consists of a glass beaker containing a small volume porous cup, sheet anode being Within the porous cup, and mercury cathode being placed in the bottom of the cell, the electrodes being in a directcurrent circuit having an ammeter and variable resistance, and the beaker placed in a cooling bath. The

catholyte is placed in the cathode chamber between the semi-permeable diaphragm provided by the porous cup and the beaker wall, and the anolyte is within the porous cup.

The reduction is effected with stirring and cooling until the reaction is substantially completed, as evidenced for example by the increased evolution of hydrogen.

The semi-permeable diaphragm of the electrolyticreduction apparatus is a diaphragm which permits hydrogen ions to pass from the anode compartment into the cathode compartment and permits the passage of anions (e.g. sulfate or chloride) from the cathode compartment to the anode compartment, but is sufficiently impermeable to prevent diffusion of the catholyte component to be reduced into the anode compartment. Various materials are suitable for use as semi-permeable diaphragms for the electrolytic reduction, and may be so used for the purpose of this invention; inter alia, unglazed porcelain, filtros and notably Alundum.

The anode for the purpose of this invention may be of any of the materials commonly used .for electrolytic reduction. Thus the anode may be of lead, lead coated with lead dioxide, platinum and graphite. On the other hand, the cathode must be of mercury in our invention.

The anolyte for the purpose of this invention may be of the compositions commonly used for electrolytic reduction, e.g. aqueous solutions of various concentrations of strong inorganic acids such as sulfuric, hydrochloric, or phosphoric acid. Preferably, the anolyte is sulfuric acid in a concentration of about 0.5% to about 80%, notably a concentration of about 10%.

The catholyte for the purpose of this invention is a non-alkaline aqueous solution of D-ribonolactone, i.e., a solution in which there is present D-ribonolactone, sufiicient acid to make the solution non-alkaline and an ammonium salt of an inorganic acid. It is most preferable to maintain the catholyte at pH 3-4. For this purpose, dilute hydrochloric acid, dilute sulfuric acid, boric acid, phosphoric acid and etc. may be preferably used. Among them, boric acid and phosphoric acid are particularly suitable for this purpose, because they also act as a buffer in the solution.

Suitable ammonium salts for this purpose are ammonium sulfate, ammonium chloride, ammonium bromide, ammonium phosphate and etc. Ammonium may form an ammonium amalgam on the surface of cathode, similar to alkali metals in the electrolysis. Ammonium hydroxide on the surface of cathode, is considered not to afiect ribonolactone so much as an alkali hydroxide due to its Weak alkalinity. Accordingly, D-ribonolactone is utilized efliciently to produce D-ribose. Furthermore, the reduction potential of ammonium ion is more positive than that of alkali metal ions such as sodium ion, potassium ion, lithium ion and cesium ion. Consequently, the formation of amalgam of ammonia with mercury on the surface of the cathode is easier than those of alkali metals. It is possible, therefore, to increase the current efficiency when an ammonium salt is used as an electrolyte instead of alkali metal salts.

Suitable concentration of the ammonium salt in the catholyte is not high, because the ammonium salt is used as the source of ammonium ion for the purpose of giving conductivity to the solution. Usually 5% is most suit able to obtain the best result.

The electrolysis may be effected at various ampera-ges; e.g., 0.5 ampere and 15 amperes, and preferably 1 to 3 ampere/(1m. of the cathode at various temperatures, in which D-ribonolactone does not decompose in the catholyte (e.g. between about 10 to about 20 C.). The high-er amperages require more eflicient cooling, however, the current densities are not a critical factor in the process of this invention.

' olyte non-alkaline, preferably at a pH When the current is passed through the cell, ammonium ions migrate to the cathode and are discharged by producing nascent hydrogen, which reacts with D-ribonolac tone in the catholyte to form D-ribose. As the catholyte tends to become alkaline during the electrolysis, dilute acid is added during the electrolysis to maintain the cathfrom about 3 to about 4.

Various D.C. voltages may be employed (e.g. volts or 110 volts), the desired amperage being obtained by suitable resistances.

The electrolytic reaction may lowing equations.

Nnix Nni+ xbe illustrated by the fol- According to the method described above, the yield of D-ribose is manifestly higher than those of prior methods.

Aside from the above stated reactions, at side-reaction of D-ribonolactone may be supposed to take place with ammonium hydroxide or ammonia to produce ribonamide on the surface of cathode, however, the ribonamide could not be isolated because of its instability in the aqueous solution. (Cf. J.A.C.S., 80,944 (1958).)

Example ml. of mercury purified by distillation was placed as the cathode in the bottom of a 300 m l. glass electrolytic cell having 36 crn. of the base. A platinum wire sealed in the glass tube was soaked into mercury as the lead wire to the DC. power source. A platinum plate having an area of 10 cm. is used as the anode and the plate is set in a diaphragm made of unglazed porcelain. 30 m1. of 10% sulfuric acid was used as the anolyte. A solution dissolving 3.3 g. of D-ribonolactone, 3.3 g. of ammonium sulfate, 2 g. of boric acid in 80 ml. of distilled water was used as the catholyte. A stirrer and the thermometer were placed in the cell and the cell was cooled with ice water from outside. Then, the electrolysis was carried out with vigorous stirring under the condition in which the temperature of catholyte was at 10-20 C. and the current was at 1 ampere. 10% sulfuric acid was dropped occasionally into the catholyte so as to maintain the pH within a range of 3 to 4 during the electrolysis. When the content of D-ribose (assayed as reducing sugar) in the catholyte reached the peak after about 4 hours, the electrolysis was stopped, and thecatholyte was separated. The solution was adjusted to litmus alkalinity With 10% sodium hydroxide, subsequently it was neutralized with sulfuric acid, and concentrated at a temperature lower than 40 C. at a reduced pressure. The residue was ex tracted with hot methanol, and the extract was condensed to a syrup containing D-ribose. Small quantities of D- ribose crystal was seeded into the syrup, then 2.5 g. of white crystal of D-ribose was precipitated. The product showed M.P. 83-85 C. which was not depressed when mixed with an authentic sample of D-ribose.

What we claim is: v

The process for preparing D-ribose, which comprises changing an electrolytic cell, having anode and cathode compartments separated by a semi-permeable diaphragm, with a non alkaline, aqueous solution of D-ribonolactone and the ammonium salt of an inorganic acid as the catholyte and an aqueous solution of a strong inorganic acid as the anolyte, the acid being substantially non-reactive with the anode material, and the cathode being mercury, passing an electric current between the anode and the cathode in the respective compartments while adding acid to the catholyte until D-ribonolactone is substantially converted into D-ribose, the electrolysis being efiected at a temperature below that at which D-ribonolactone decomposes in the catholyte, and recovering D-ribose from the catholyte.

FOREIGN PATENTS 118,130 3/1958 U.S.S.R.

JOHN H. MACK, Primary Examiner. H. M. FLOURNOY, Assistant Examiner. 

